We showed altered expression of Hsp40s, nucleotide exchange factors (NEFs) and several TPR-domain proteins affect prion propagation in wild type and Hsp70 mutant cells. We also identified several Hsp40 and TPR protein mutant alleles that impair or enhance prion propagation. Together our data imply that many, if not all the observed effects of co-chaperones on prions are mediated by their regulation of Hsp70 activities. In many instances the same conditions producing a significant effect on one prion have little or no effect on a different prion, pointing to a prion preference or specificity of the Hsp70/co-chaperone partners. Targeting the two major yeast Hsp40s, we have identified mutations that alter prion propagation in both positive and negative ways. So far we have determined that at least some of the effects are dependent upon interactions with particular Hsp70 isoforms. This work identifies specific Hsp40/Hsp70 pairings that influence prion replication and growth processes in defined ways.[unreadable] We have purified wild type and mutant Hsp40s to homogeneity and will continue assessing functional interactions between these co-chaperones and Hsp70 in vitro. We are specifically interested in how they influence physical properties and propagation of amyloid, or prevent protein aggregation and refold denatured proteins. By uncovering suppressive effects of nucleotide exchange factors on ability of a mutant Hsp70 to antagonize prions, we found that these co-chaperones also could influence prion propagation through their ability to regulate Hsp70. We have purified one of these proteins and are in the process of purifying another to determine how specifically altering another part of the Hsp70 reaction cycle affects prions.